Heretofore, e.g. a polyether monool, a polyether polyol, polyester ether polyol or a polycarbonate ether polyol (hereinafter referred to generally as “a polyether”) to be used as raw material for e.g. a polyurethane elastomer, an adhesive, a coating material or a sealant, was produced by (co)polymerizing a compound selected from the group consisting of an alkylene oxide such as ethylene oxide or propylene oxide, caprolactone and an acid hydride, with an initiator having an active hydrogen atom. As a typical polymerization catalyst applicable to such a polymerization reaction, a double metal cyanide complex catalyst (hereinafter referred to also as “DMC catalyst”) has been known. A DMC catalyst is a catalyst containing an organic ligand and a metal salt, and a compound having an organic ligand, water and zinc chloride coordinated to zinc hexacyanocobaltate (Zn3[Co(CN)6]2) is typical.
By employing tert-butyl alcohol as an organic ligand, it is possible to remarkably prolong the life span of a DMC catalyst, and thus, it has become possible to produce a polyether with a small amount of a catalyst to be used. However, even though the amount of a DMC catalyst contained in a polyether as a final product is such a small amount, there may still be a problem of e.g. a poor storage stability of an isocyanate group-containing prepolymer produced by reacting a polyisocyanate compound with the polyether obtained.
Heretofore, many proposals have been made with respect to the process for purifying a polyether produced by using a DMC catalyst. A purification process is broadly classified into a process of directly filtrating and separating fine particles of a catalyst residue contained in a polyether and a process of subjecting a catalyst to chemical treatment, adsorbing a decomposed product of the catalyst with an adsorbent, followed by filtration and separation. Proposed is a process of directly separating an unpurified polyether by filtration by a ceramic membrane having a pore diameter of from 5 to 100 nm or a microseparation membrane made of a polyvinyl fluoride, so that the remaining amount of a metal would be less than 1 ppm. However, in such a process for membrane separation of a catalyst as fine particles, clogging of the separation membrane tends to occur due to such particles, whereby there will be a problem that the filtration time tends to be long.
Many processes have been proposed, in which a DMC catalyst is deactivated with a basic substance and neutralized with an acid, and then a metal derived from a catalyst is separated by filtration from a polyether by using an adsorbent and a filter aid (for example, the following Patent Documents 1, 2, 3, 4, 5, 6, 7 and 8). Further, such Patent Documents also disclose use of an adsorbent and a filter aid having specific characteristics and specific particle diameters, but each case is directed to a method in which a basic substance is used for decomposing a DMC catalyst. Accordingly, a complicated purification step is required to sufficiently remove a basic substance which affects the reaction with an isocyanate or an acidic substance to be used for neutralizing the basic substance, from the polyether, and further there is a problem that a large amount of waste is produced since a large amount of an adsorbent is used.
Proposed are a process (Patent Document 9) in which, in the presence of water, a polyether containing a DMC catalyst is subjected to heat treatment to deactivate the catalyst, followed by crystallization by using a mineral acid or adsorption by an adsorbent, and then by dehydration under reduced pressure and further by separation by filtration, a process (Patent Document 10) in which, in the presence of water and zinc oxide, a polyether is subjected to heat treatment to deactivate a catalyst, the catalyst is then adsorbed by an adsorbent, and then dehydrated under reduced pressure, followed by separating the adsorbent by filtration (Patent Document 10), and a process (Patent Document 11) in which, in the presence of magnesium oxide or a combination of magnesium oxide and water, a catalyst is deactivated by heat treatment, adsorbed by an adsorbent, and then dehydrated under reduced pressure, followed by separating the adsorbent by filtration. In such processes, no basic substance is used, and therefore no acid-neutralizing step and cumbersome purification step are required. Further, as an inorganic adsorbent, synthetic aluminum silicate, synthetic magnesium silicate, activated clay, acid clay and a mixture thereof are disclosed. However, in a purification process disclosed in Patent Document 9, there will be a problem that it is impossible to obtain a sufficiently high filtration rate at the time of filtrating a highly viscous polyether containing fine particles derived from a catalyst deactivated and decomposed. Further, its Examples disclose that from 5 to 10 ppm of a metal derived from a DMC catalyst remains in a polyether after filtration, and the removal of the catalyst is not fully satisfied. Further, in Patent Documents 10 and 11, large amounts (4% to a polyether) of an inorganic deactivator and an adsorbent are required to reduce the amount of a metal remaining in a polyether to a level of from a few ppm to 1 ppm, and therefore there will be a problem that the waste amount of a cake including the adsorbent and the polyether adsorbed thereon becomes large.
Further, a method (Patent Document 12) is proposed for removal of catalyst and deodorization, from a polyether containing a DMC catalyst, by using, as an adsorbent, sepiolite which is microcrystalline magnesium silicate. Such a purification step is simple, but even when a polyether is purified by using at least 1 mass % of an adsorbent, the amount of a metal remaining in the polyether after the purification is at least 1 ppm, and therefore the purification level is not fully satisfied.    Patent Document 1: JP-A-1-229035    Patent Document 2: JP-A-2-242821    Patent Document 3: JP-A-2-289618    Patent Document 4: JP-A-3-88823    Patent Document 5: JP-A-3-115430    Patent Document 6: JP-A-4-197406    Patent Document 7: JP-A-4-197407    Patent Document 8: JP-A-4-268329    Patent Document 9: JP-A-3-88824    Patent Document 10: JP-A-2002-201263    Patent Document 11: JP-A-2002-212280    Patent Document 12: JP-A-2003-342362